Background: The practice of installing relays
for the purpose of controlling Headlight power is common on new cars.
This practice allows designers to install dainty, decorative, in many cases,
illuminated, low current switches, or even multifunction switches,
such as for instance, the stalk which also controls the Directional Indicators,
or to incorporate fancy features like pathway lighting, where the computer
energizes the relay for thirty seconds in the dark after you've left the vehicle
so you can find your way to the front door (I find myself not trusting this and
watching the car through the window until they turn off...) .
These cute, but wimpy switches are not intended to, nor could they ever, handle
the full current of the Headlights, but they can easily control a comparatively
miniscule relay control current, for a relay located elsewhere.

Compare this to old vehicles, where switches in the
dashboard were
large and simple, and which could easily carry the full load current. I
guess Relays used to be expensive and wire cheap, now it seems to be the other
way around!

The point is, Relay Headlight control is fitted because
of the power constraints of switches, NOW, because it IS inherently better
design practice for new designs. But the argument for Relays (as shown in Hella's documentation. See
Sw-Em Tech Article: Lighting, Relay Controlled Headlights)
does not necessarily apply to older vehicles whose dashboard switches were perfectly
capable of handling the full load current! If you have weak
headlights, it is NOT time to reengineer the vehicle wiring, it IS time to bring
its performance back the new level!

To help with this, I need to point out a subtle built-in weaknesses on a 122
and non-British harness* 1800s which had Hex-Connectors routing power to
Headlights, which can cause the headlights to receive a reduced voltage!...but
again, the remedy is not to reengineer the whole system and install Relays...the answer
is a lot simpler!

* British harnesses use different "Bullet" style connectors.
These will be considered in a separate,
future article!

Excerpt of 122 Wiring Diagram showing Junction Unit at Red.

The innocent looking connection at the Junction Unit...I call it a Hex-Connector...hints at the root of the issue. If we
look closer at positions 4, 5, and 6, it is apparent that wiring for both Right and
Left Headlight circuits comes from one point on the output/load side of
the Hex-Conn...but it is not by way of both wires crimped into one
terminal as one might expect. Instead, each wire crimped into its own (male) connector, and
both of these connector then stacked as they are inserted into the housing...and
herein lies the potential weakness. As the mating Hex-Conn is then connected, one of
these Bifilar Male Terminals has a connection advantage with the female
terminal...read on!

Hex-Conn Bifilar Male Terminals, removed from Housing (an up-close and
personal inspection of both terminals and housing with a lupe confirms
manufacturer of these components was quality US industrial connector specialist
AMP):

Gray, Low-Beam Headlight wires (from position 4 of Hex-Conn).

Bifilar Male Terminals In-Situ. When seated home in Housing,
retention clips face out in opposite directions to both pick up Housing edge:

At Blue, both Bifilar Male Terminals are visible (one is intentionally slightly
askew of the other for visibility). At Green, retention clip of one of the two
middle terminals has been pushed down to allow partial withdrawal
(visible is plenty of gray oxidation on all surfaces of terminals,
including between the two center terminals). At Yellow, retention
clip is visible in normal position, preventing terminal withdrawal.

All of the terminals are exhibiting gray
non-conductive oxidation
(Correction: Tin-oxide on
the Tin plating is conductive!), and thanks
to the excellent, forgiving design of
the .250" terminals in general (see: Reference Information -
The Lowly .250" Push-On
Terminal), where the female
terminal cuts through the oxidation on the male terminal with two high pressure, spring-loaded edges
(see below), as connectors are mated, normally this would not be a problem, but
because the Hex-Conn uses a bifilar design at pins 4, 5, and 6, the lower male
connector, facing the straight only, non-cutting side of the female terminal, is much
more likely to have a poor connection because it is not subjected to a
high-pressure cutting edge, but only a low-pressure flat oxidized surface of the
lower corroded male terminal! The end result is that this lower terminal
is likely to have a high-resistance connection to the voltage source...you can guess the
rest!

At Green, a high-pressure spring-loaded edges cut through oxidation of upper
male terminal!.
At Red, low pressure (because of high surface area) to lower male terminal is much less likely to cut
through oxidation.

Looking closer at the actual terminals, shown below are the bifilar
terminals for the Red (High Beam) power wire (station 6 of Hex-Conn), removed
from housing. At Green, shiny cut marks through oxidation layer are
apparent on what was upper male terminal, from where edge of a female
terminal was test-mated. At Orange on what was the lower male terminal,
shiny cut marks remain from where retention Pimple (opposite of Dimple?) slid
over surface, but none is visible at or above retention hole, indicating that
when female terminal was in home-position, there was not necessarily high
pressure, cutting contact with base-metal of terminal.

At Green, shiny areas on male terminals shows where two edges of female
terminals have cut through corrosion.
Blue shows variations in female terminal design, particularly separation of
cutting edges.

The solution to dim/yellow Headlights due to this in-line resistance is
simple, before considering changing your Headlight wiring system over to
include Relays: One at a time, push the retention clips on the male
terminals of Hex-Conns to defeat their function, remove the terminals, inspect**
and clean terminals to bright shiny metal, add a film of
ACZP,
and reassemble into the Housing. Done!

** The crimped joints of terminals likely
don't look much better than the flat area. Ideally, this would be a
good place to solder the wire into the terminal to minimize resistance, and
make a Gas-Tight-Joint. Replacement terminals are available from
GCP, (see Reference Information - Replacement Terminals), but they
can even be reused! A careful decrimping, cleaning of crimp area of terminals
to shiny metal with a wire brush, as well
as cutting back the wire to expose a new shiny copper area of the wire, then
gently recrimping AND soldering of the old terminal will also give an optimum
connection. [Possibly a memorable Quote: "When I want the best crimped connection, I solder
it!" R. Kwas]

The Lowly .250" Push-On Terminal is a
little
marvel of design and function. It has been engineered to allow a highly reliable, semi-permanent,
high current connection to be quickly and effectively made, which still can be
disconnected by hand with minimum effort. It is inexpensive, and simple to be
installed onto wires at the factory in a high volume production environment with
automated tools, as well as being able to also be installed one at a time with
hand-tools, by Joe-weekend-vintage-Volvo-mechanic....

...the two curved sides
of the female contact are effectively a single turn spring (with variations, see
below), which keep the two connection areas under constant preload...this
accommodates any production dimensional variations, as well as thermal
dimensional changes and vibration in-service very well…any variations, are
continuously taken up by the constant preload!

.250" terminal conceptual drawing showing a typical version on the left,
and another version, with even higher contact pressure (because of
decreased contact area), on right.
Spring preloaded electrical contact is made at three places, two at edges on
topside, one at retention dimple on bottomside.

...they make a very good
mechanical and electrical connection, with the cutting action during connector
mating assuring two virginal metal surfaces. This can be considered a
Low-Contact Area/High Force situation. Add the benefits of long-term
protection by ACZP,
and we have a semi-permanent electrical connection very well suited for,
automotive applications…again, good engineering at work!

Special versions of 1/4" contacts:

Fully insulated 1/4" crimp terminals. Once mated, in an in-line
connection,
there are no exposed conductors to make inadvertent contact with
anything!

...but beware...there are ways to get it wrong, too...in making harnesses for
some of the SwEm Kits, I like the complete coverage of the fully insulated,
female crimp-on contacts (...nothin' but the best for my customers!)...shown
below is one of these female terminals being pushed on the male terminal of a
Brake Light Switch...wrongly! It is possible to incorrectly
insert these onto the mating terminal...this might make a good electrical
connection today, but might become intermittent or even work its way apart
tomorrow under vibration, because the mechanical connection does not have the
retention force of the engaged dimple/pimple.

Insert carefully and correctly when using these terminals!
(Shown clean below for detail, but in service, should have a protective coating
of ACZP!)

1/4" female terminal misinsertion...possible because slots on
either side of the actual terminal allow this.
This is an example of poor design! This should not be possible, as an
example of poka-yoke!
In kits, this connection is gooped up with ACZP,
but that would obscure what we need to see here...

"Problem solved, was oil/grease buildup on both electrical contacts. No
disassembly required."

My response: "Be aware...if "oil/grease buildup on both
electrical contacts" is enough to prevent electrical contact, the spring preload
of push-on terminal (which should be present!) must not have been very
good, because by design, pushing on terminals should cut through oxidation and
surface contamination to allow contact of clean metal surfaces. This means after
cleaning terminals of switch, you may want to (at the very least) squeeze
terminals to restore some preload, and (at the very most) replace crimps with
new! ...and of course: Apply ACZP!"

You are welcome to use the
information here in good health, and for your own non-commercial purposes, but
if you reprint or otherwise republish this article, you must give credit to the
author or link back to the SwEm site as the source. If you don’t, you’re just a
lazy, scum sucking plagiarist, and the Boston Globe wants you! As always, if
you can supply corrections, or additional objective information or experience, I
will always consider it, and consider working it into the next revision of this
article...along with likely the odd metaphor and probably wise-a** comment.